Because of the many physical property requirements of mirrors for high technology applications (e.g. lasers), both a variety of materials and designs have been employed in attempts to optimize the particular properties necessary for a composite used for these applications. For example, while a laser mirror must have the requisite reflective properties, cost and availability of materials as well as ease of fabrication are also important factors. Such mirror should also desirably have low density for each of use in the types of apparatus where they will be used, but without porosity. Furthermore, such mirrors ideally should have high elastic stiffness and high strength along with high fracture toughness. And stability is of the utmost importance both from the point of view of the fine resolution-type work environment the mirrors will be used in, and the inaccessibility of the apparatus which these mirrors would be used in, for example, outer space applications. These stability properties includes low thermal expansion, high thermal conductivity, and environmental stability. Environmental stability includes such things as dimensional stability and mirror integrity regardless of moisture conditions, vacuum conditions or ultraviolet light exposure, and mirror integrity and dimensional stability at both high and low temperatures. Because high technology mirrors are used for space applications weight is also an important factor.
Carbon fiber reinforced glass and glass ceramic composites are excellent materials for these applications in that they possess exceptional dimensional stability and high structural toughness. See commonly assigned U.S. Pat. Nos. 4,410,394 entitled "Methods of Making Cooled Thermally Stable Composite Mirrors" and 4,464,192 entitled "Molding Process for Fiber Reinforced Glass Matrix Composite Articles". In addition, these composites can be fabricated more rapidly than the traditional monolithic glasses and glass-ceramics used for large mirror applications because of their ability to be rapidly heated and cooled without cracking and their superior ability to be machined. The fabrication of large light weight mirrors is, however, also a structural problem in that sufficient structural rigidity must be achieved at a minimum weight. It has been typical in the past to fabricate such articles by the bonding of top and bottom composite places to an inner core. This process, however, results in distinct bond lines, between surfaces and core, which can act as regions for delamination.
Thus there has been a constant search in the art for fiber reinforced structures and methods for making the same, particularly composite mirror configurations.